Field of the Invention
The present invention relates to a device for detecting the presence of one or a plurality of specific substances and for producing a suitable output signal upon the detection thereof.
Related Art
Photocells have often been used in substance detecting systems. U.S. Pat. No. 3,487,069 discloses a refractometer with compensating photocells. A light beam is reflected from a prism-liquid interface onto three groups of photocells. One of the photocells measures light at the angle of reflection and the other photocells compensate for unwanted light which is reflected onto the measuring photocell. A difference in light intensity falling on the photocells causes a varying output current which is detected by a microammeter. The photocells are used for detecting the edge of a light beam, i.e., a sudden change in light intensity that occurs at the critical angle. When liquids with varying indices of refraction are used, the critical angle changes. As the index of refraction of the liquid increases, the critical angle also increases, and the bright area decreases. Thus, the output of the photocells is effectively decreased. The "beam edge" is detected by measuring the electrical difference in output between the reference photocell and the pair of photocells. This measurement is approximately proportional to the difference in illumination between the cells. Accordingly, the electrical output, which is proportional to the deflection to the microammeter, is susceptible to errors from stray light, drifting of characteristics in system components, and fluctuations in supply voltage.
U.S. Pat. Nos. 2,355,014, 2,359,787, 3,540,025 and 3,751,672 disclose single photocell devices which utilize the principle of total reflection and a single photocell to indicate the presence of a substance or its index of refraction. These systems operate on the principle that the presence of a substance on an optical interface would result in a larger critical angle than with air. This increases the area having a lower reflection intensity and indicates the presence of the detected substance by a decreased light intensity on the photocell. This approach is particularly prone to erroneous readings, because it is susceptible to light variations, dirt in the optical system, aging of components, etc. Further, erroneous results will likely be produced for substances of slightly different indices of refraction, such as water and ice.
U.S. Pat. Nos. 3,045,223, 3,932,038, 3,94,131, 4,379,227 and 4,538,064 utilize the darkening effect or light-type effect of a substance to detect its presence. This effect is used to change the intensity of light falling on a photocell from some light source.